Official records released into the public domain on 4 January 2005 as part of the Freedom of Information Act 2000 only date back to 3 June 1929,[5][6] when A.E. Banham at the Treasury authorised the Office Keeper "to spend 1d a day from petty cash towards the maintenance of an efficient cat".[7] In April 1932, the weekly allowance was increased to 1s 6d. By the twenty-first century, the mouser was costing £100 per annum.[8] The cats do not necessarily belong to the Prime Minister in residence and it is rare for the Chief Mouser's term of office to coincide with that of a Prime Minister.[9] The cat with the longest known tenure at Downing Street is Wilberforce, who served for eighteen years under Edward Heath, Harold Wilson, Jim Callaghan, and Margaret Thatcher.

The post is currently held by Larry, since 15 February 2011.[1] The departure of the last incumbent, Sybil, was in January 2009. Sybil, who began her tenure on 11 September 2007, was the first mouser for ten years following the retirement of her predecessor Humphrey in 1997. Sybil was owned by the then Chancellor of the ExchequerAlistair Darling, who lived in 10 Downing Street while the then Prime Minister, Gordon Brown, lived in the larger 11 Downing Street.[10][11] It was reported that Sybil did not settle in London, and returned to Scotland to live with a friend of the Darlings.[12] Sybil died on 27 July 2009.[13][14]

In January 2011 rats were seen in Downing Street, "scurrying across the steps of Number 10 Downing Street for the second time during a TV news report," according to ITN.[15] There being no incumbent Chief Mouser at that time, the Prime Minister's spokesman said there were "no plans" for a cat to be brought in to tackle the problem;[16] however, the following day, newspapers reported that the spokesman had said there was a "pro-cat faction" within Downing Street, leading to speculation that a replacement might indeed be brought in to deal with the problem.[16] On 14 February 2011, it was reported that a cat called "Larry" had been brought in to address the problem.[17] The London Evening Standard reported that the cat had been selected by David Cameron and his family, from those at Battersea Dogs & Cats Home.[17]

In September 2012 it was reported that Prime Minister David Cameron had sacked Larry from the post of Chief Mouser, in favour of Chancellor George Osborne's tabby, Freya, as the new Chief Mouser to patrol Numbers 10, 11, and 12. Some sources described the new arrangement as a "job share" to avoid any hurt feelings.[18] Chief Mousers in the past have overlapped, or been phased in – though the position can and has remained vacant for extended periods of time. Larry is the only Chief Mouser listed on the official web site for Number 10.[1] In November 2014 Freya was exiled from Downing Street leaving Larry as the Chief Mouser.[19]